This invention generally relates to coherent imaging methods which apply a phased array antenna. In particular, the invention relates to beamforming techniques for use in ultrasound imaging systems.
A medical ultrasound system forms an image by acquiring individual ultrasound lines (or beams). The lines are adjacent to each other and cover the target area to be imaged. Each line is formed by transmitting an ultrasonic pulse in a particular spatial direction and receiving the reflected echoes from that direction. The spatial characteristics of the transmitted wave and the characteristics of the receive sensitivity determine the quality of the ultrasound image. It is desirable that the ultrasound line gathers target information only from the intended direction and ignores targets at other directions. Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal zone of each beam being shifted relative to the focal zone of the previous beam. In the case of a phased array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal zone can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe. is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
In a typical ultrasound system the beamformer control is a significant contributor to the performance and cost of the system. Dynamic receive delay and apodization control must be generated for each beam and channel at a high rate. Often as many as 512 beam channels are required for a high-end system with an update rate as high as 10 MHz. Calculating delays and apodization for an ultrasound beamformer during dynamic reception requires complex calculations including transcendental functions. Normally image quality is traded off for cost by using second or third-order approximations for these functions. Additionally, large memories are frequently used to store pre-calculated controls for predetermined beam positions and parameters. Using stored pre-calculated values limits the ability of the system to optimally readjust beam position or parameters dependent on scanning situations. Thus image quality is compromised to achieve faster control response times.
There are a wide variety of beamformer control designs currently in use. All use some combination of large parameter random access memories (RAMs), state machines, complex calculations and approximations. Thus they fall short in one or more areas: agility, cost or precision.
Agility means the ability to change the beamforming setup for an entire imaging configuration in a time much less than 1 sec. where the setup includes vector phase center (aperture) positions, steering angles, f-numbers, and focal positions, and the imaging configuration includes all vectors (beams) displayed on the imaging console. Agility also means the ability to begin receive delay computation in mid-flight, i.e., immediately before a deep region of interest.
A new beamforming architecture which does not require large memories for storing pre-calculated beamforming values, which works equally well with transducer arrays of any geometry, and which is agile, precise and low in cost was disclosed in U.S. Pat. No. 6,123,671. That patent discloses an architecture for calculating beamformer time delays and apodization values in real-time by using a cordic rotator (hereinafter xe2x80x9ccordicxe2x80x9d), a ""simple multiplier-less device used for polar-Cartesian conversions. The use of the cordic to directly calculate the root-sum-of-squares without approximation or complex logic;provides ideal performance and flexibility at low cost. The system may quickly re-optimize the beam positions and parameters without the need to recalculate large pre-stored memories or use rough approximations. More specifically, the cordic rotators are utilized calculate ultrasonic beamforming apodization weightings and time delays. Apodization weightings and time delays are calculated in a real-time, distributed and pipelined manner without the need for large memories or complex state machines. There are no inherent approximations. Precision may be easily controlled to be within the accuracy of the delay apparatus. When a transducer array is attached to the ultrasonic imaging system, the element positions are written into relatively small distributed memories. The element positions do not need to be reloaded except when the array geometry is changed. While imaging, a small number of parameters are broadcast before each transmission and during each reception. These parameters may include: focal position(s), multiplexer state(s), aperture position(s), aperture size(s) and vector angle(s). Simple logic, including cordics, combines these parameters to produce beamforming time delays and apodization. Focal position and apertures may be changed very quickly for an entire image, in milliseconds. The receive focal positions do not need to start at the skin-line or follow a straight line, as required by state machine approaches. Reception focus can begin in mid-flight, and be warped to compensate for parallel beamforming methods. In addition, the array geometry may take any form; it is not restricted to the conventional linear, convex and phase arrays.
Many modern ultrasound scanners use curved probes to increase field of view and sensitivity. One of the problems with curved probes is the delay error caused by the refraction of the ultrasonic waves within the lens of the probe. A known beamformer calculates the focusing delays by assuming the ultrasonic waves travel a straight line from the center of an element to the focus at an average speed assumed to be 1.54 mm/xcexcsec. In reality the probe lens affects both the direction and speed at which the ultrasonic waves travel. Thus the aforementioned known beamformer has a refraction delay error which is particularly severe for curved probes. Any delay error will degrade the focusing of the ultrasound scanner.
Another known beamforming system attempts to correct for the refraction delay error by using an adjusted xe2x80x9cfalsexe2x80x9d radius of curvature (ROC) for curved probes. This false ROC is only a partial solution as it only corrects the refraction delay errors at one range. There is a need for a method to correct for the refraction delay errors for all ranges.
The present invention is directed to a method and an apparatus for correcting refraction delay errors on curved probes for all ranges. In accordance with the preferred embodiment, an agile beamformer is utilized. However, the angular correction method of the invention could be implemented in other forms. The idea is based in part on the observation that, if the angle xcfx86 from the normal of an element to the focus can be determined, then a delay error correction can be indexed using this angle. In the prior art (false ROC) beamformer previously mentioned, it would be computationally expensive to calculate the angle xcfx86 as a function of range. But in the agile beamformer, determining the angle xcfx86 is a relatively straightforward process.
In accordance with the preferred embodiments of the invention, the agile beamformer corrects for the delay errors caused by refraction for all ranges by using an angular correction method. The angular correction method is efficient in that it uses the inherent property of the cordic rotators to calculate the only range-dependent variable required for the correction. This means the additional hardware required to calculate the corrections is minimal, as the remaining correction variables are vector and range independent. Furthermore, each delay calculator within the agile beamformer handles 16 channels, further decreasing the hardware impact of an angular correction scheme. The angular correction method in accordance with the preferred embodiment uses a lookup table (LUT) of 256 possible delay adjustments and reduces the delay error to less than 10 nsec for two known probe models.